1. Field of the Invention
This invention relates to heat exchangers and, more particularly, to motor vehicle heat exchangers utilizing grommets in the tube-to-header joints and tanks welded to the header.
2. Description of Related Art
Heat exchangers, particularly those utilized in motor vehicles, may be liquid-to-air heat exchangers, (e.g., radiators for engine coolant, air conditioning condensers and evaporators, and oil coolers) or may be air-to-air heat exchangers (e.g. charge air coolers). Liquid-to-air and air-to-air heat exchangers are typically composed of an inlet tank or manifold, an outlet tank or manifold, and a large number of tubes extending between the tanks or manifolds which carry the fluid to be cooled. Headers are normally provided on the tubes for mechanical attachment and fluid connection to the tanks. Fins attached to the tubes transfer heat between the liquid or gas inside the tubes and the ambient atmosphere outside. A mechanical framework or structure is usually included to provide structural strength to the assembly and to provide means for mounting the unit to the vehicle or other machinery on which it is used.
The tubes utilized may be either round or oval, or may be oval with circular ends. Prior art methods of welding tube-to-header joints are disclosed, for example, in U.S. Pat. No. 5,407,004, the disclosure of which is hereby incorporated by reference.
In use, heat from the hot liquid or air within generally causes the tubes to expand and grow in length due to thermal expansion. Since the tanks or manifolds are fixed with respect to each other by the unit framework or structure, the growth in length of tubes places high mechanical stresses on the tanks and the associated headers, particularly in the area of the joints between the tubes and headers. In addition, the pressure of the hot liquid or hot air within the heat exchanger tends to distort the tanks or manifolds and headers, creating further stresses on the tube-to-header joints. The combination of stress resulting from thermal expansion and internal pressure can result in early failure of heat exchangers. Cracks in the joints between the tubes and the headers or in the tubes immediately adjacent to these joints are the most common mode of failure. Many approaches have been taken to avoid heat exchanger failures due to thermal expansion and internal pressure. Most approaches fall into one of two categories: 1) those which improve the strength of the areas prone to failure and 2) those which provide resilience in the areas prone to failure. Approaches which provide resilience have appealed to designers because they provide a solution to the stresses of thermal expansion and internal pressure with a greater economy than any approach which must provide more material to achieve an improvement in strength.
Engine cooling radiators for vehicles have sometimes been designed with resilient tube-to-header joints. Locomotive radiators have been manufactured by the assignee of the present invention for over thirty (30) years using headers of special resilient design. In this design, metallic headers have oversized holes or openings in them to receive oval brass tubes extending from the radiator core. Within the openings in the header there are placed oval brass ferrules. These ferrules are bonded to the header by molded silicone rubber. The ferrules are then soldered to the core tubes extending therethrough to form a leak-free, resilient joint between the tubes and the headers. Fins of the flat plate-type design have collars fitted around the tubes. The headers are mechanically attached to tanks, such as by bolting. While this has been an extremely effective design under typical operating conditions for locomotives, it is expensive to produce.
In the 1970's, radiators for automobiles were produced which utilized round aluminum tubes, aluminum plate fins, aluminum headers and plastic tanks. A sheet of molded rubber provided resilient grommets at each tube hole in the header, and also provided a gasket for sealing the headers to the plastic tanks, which were attached to the headers by means of crimped tabs on the headers. The insertion of the tubes into the rubber grommets in the header holes compressed the rubber of the grommets providing a resilient sealing attachment of the tubes to the headers. However, considerable force was required to insert all the core tubes into the header holes simultaneously. This design was limited to relatively small units because of the problems of core and header distortion during assembly and because of the close tolerances which were required to accomplish the mating of the core tubes to the header with the desired amount of grommet compression.
Other radiators have also utilized rubber grommets in their tube-to-header joints. These radiators have been designed around individual finned tubes having round ends and oval cross-sections which are finned along most of their length. As in the previous design, sealing of the tubes to the header was accomplished by compression of the grommets between the tubes and the header. However, in this alternative design, the tubes were assembled to the headers individually thereby avoiding high assembly forces. This allowed the construction of very large radiators for heavy construction equipment. However, it has been found that the use of tubes with round ends limits this design to cores having rather wide tube spacing, which results in relatively poor thermal performance compared to most radiator core designs.
U.S. Pat. Nos. 4,756,361 and 5,205,354 describe a radiator which utilizes circular tubes and tube ends which extend through silicone rubber grommets which are disposed in openings within a header plate. U.S. Pat. Nos. 5,052,475 and 5,226,235 disclose use of circular grommets to seal circular tubes into soldered tanks and welded tanks, respectively. British Patent No. 29,777 discloses the use of round tubes and grommets with a tube plate cast integrally with the header.
Currently, air-to-air heat exchangers using brazed aluminum cores having oval tubes are being produced commercially. Aluminum headers having oversized oval openings are welded to cast aluminum manifolds. Oval silicone rubber grommets, otherwise similar to those described in the aforementioned '361 patent, are inserted into the openings in the headers of the welded tanks.
In the aforementioned '361 patent, the headers are intended to be connected to inlet and outlet tanks by means of bolting with a sealing gasket, or other similar means. U.S. Pat. No. 5,226,235 describes a radiator made up of a unitary core consisting of tubes and fins, connected to tank assemblies which consist of welded tank and header assemblies into which resilient grommets have been inserted after welding. This '235 patent provides advantages over the '361 patent in that the welded tank and header assembly is more compact than one assembled by bolting with a sealing gasket. In addition, a welded tank design is quite flexible and can be quickly made up to suit any application. However, a difficulty with the '235 patent is that it requires that all welding of the tank/header assembly is to be completed before assembly of the resilient grommets to that assembly. This is stated to be necessary to prevent heat damage to the resilient grommets. However, it makes assembly of the unitary core assembly to the tank/header assemblies particularly difficult, since one can only see one side of the grommets and access to the other side of the grommets to aid in assembly is denied. For instance, it would aid insertion of the core tubes into the grommets if the tube ends were fitted with removable bullet noses. However, in the one-piece tank/header design, retrieval of the bullets after insertion is practically impossible.
Additionally, access to the side of the grommets remote from the tube insertion side can be helpful to tube insertion in other ways. For example, supporting or backing-up the remote side of the grommet during tube insertion makes tube insertion easier and prevents pushing out of the grommets. This is, of course, impossible with the one piece tank/header assembly described in the '235 patent.
It would be advantageous to have the critical tube-to-header joints made and inspected with full access to both the airside and fluid side of the joints. The core assembly can be separately tested and corrections can be made to the tube-to-header joints, if necessary, before the tanks are attached.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved heat exchanger and method of making such heat exchanger which utilizes grommets in tube-to-header joints, which header may be welded to the tank after insertion of the grommets.
It is a further object of the present invention to provide an improved method of assembly of a heat exchanger in which the core assembly can be separately tested and corrections can be made to the tube-to-header joints, if necessary, before the tanks are attached.